The present invention relates to stud unit and skid-proof tire having the stud unit for preventing vehicles from slipping on snow- or ice-covered roads.
Conventionally, snow tires having multiple studs on the outer circumferential face are used as skid-proof tires.
However, studs of the snow tires grind surfaces of roads when vehicles using the snow tires run on a road whose surface is dry and exposed. Powdered dust, which is worn from roads by grinding with studs of the snow tires, is blown up in the atmosphere and causes air pollution. The powdered dust is harmful for our health because it includes such harmful heavy metals as cadmium, lead, and the like. Further, traffic signs painted on roads are worn off by studs of the snow tires, which will be a factor in traffic accidents and a heavy expenditure for repairing the signs will be required. Then, to avoid the above noted disadvantages, melting snow and ice on roads by spraying calcium chloride and the like is attempted but roadside trees are blighted and secondary pollution, such as water pollution, occurs.
To avoid all these disadvantages, a skid-proof tire having stud units was disclosed in the Japanese Patent Provisional Publication (Kokai) Gazette No. 59-186704. Each stud unit shown in FIG. 13 has a cylinder 2 and a rubber elastic member 3 therein. A stud 5 is passed through a hole 3a, which is bored in the center of the rubber elastic member 3. The rear end of the stud 5 is engaged with an inside face of rear wall 2a of the cylinder 2. There is formed a large-neck section 6 at the front end of the stud 5, and the large-neck section 6 projects forward from the front opening of the cylinder 2.
Therefore, a surface of a road is not damaged while a vehicle using the tires runs at fixed speed because the stud 5 is pushed in the cylinder 2 against the elasticity of the elastic member 3.
When the vehicle is braked, accelerates, or reduces speed, a force, which is caused by the friction between the tire and the surface of the road, acts on the stud 5 contacting the surface of the road in the horizontal direction corresponding to the rotation of the tire, so that the stud 5 is inclined in the cylinder 2 and the jaw section of the large-neck section 6 engages the rim of the opening of the cylinder 2 to keep the stud projected. Thus, slippage can be prevented.
However, the above noted tire has following disadvantages:
(1) It is necessary to design the diameter of the shaft of the stud 5 large because a heavy force in the inclining direction acts on the stud 5 when the vehicle accelerates or is braked. But, if the shaft of the stud 5 is bigger, the elastic member 3 and the cylinder 2 must be bigger, so that each of the stud units must be heavier.
(2) When the stud 5 is inclined, the elastic member 3 is sometimes clipped as in a jaw between the underside of the large-neck section 6 and the rim of the opening of the cylinder 2.
(3) Because the durability and stability of the elastic member are low, the elastic member is apt to be permanently deformed by frequent inclination of the stud 5, so that the stud 5 sometimes will not return to the center.
(4) To engage the rear end of the stud 5 with the outside face of the rear wall 2a of the cylinder 2, it is necessary to squash the rear end of the stud 5 and to fix the rear wall 2a of the cylinder 2 by welding or the like, so that the assembling steps are increased and troublesome.